1. Field of the Invention
The present invention relates generally to the technical field of automated tools used for integrated circuit fabrication and, more particularly, to automated reticle handling tools.
2. Description of the Prior Art
Presently, conventional processes for manufacturing integrated circuits (“ICs”) include a number of process steps in which a surface of a semiconductor wafer is first coated with a thin layer of a photo-resist material after which the photo-resist material is irradiated with short wavelength light to form a latent image of a pattern in the photo-resist layer. A subsequent processing step develops the latent image thereby leaving patterned photo-resist material on a wafer's surface. In processing a semiconductor wafer to fabricate ICs, the preceding procedure for establishing patterned photo-resist material on a wafer's surface may be repeated dozens of times. The pattern formed in the photo-resist layer on a wafer's surface generally differs for each of the dozens of photo-resist exposures performed during IC fabrication. Thus, fabricating a particular type of IC may require a dozen or more reticles each having a different pattern.
In irradiating photo-resist material, the short wavelength light used in forming the latent image passes first through a reticle before impinging upon the thin layer of photo-resist material. In general, reticles used in IC fabrication are made from a thick, planar, rectangularly or square shaped pieces of glass. The reticle is opaque in those areas of the pattern where the reticle blocks the short wavelength light from impinging upon the thin layer of photo-resist material. A reticle's opaque pattern is usually formed by a layer of patterned metal, e.g. chromium, coated on one surface of the piece of glass forming the reticle.
Because reticles are high precision optical devices, they are comparatively expensive. Presently each individual reticle can cost between $5,000.00 and $30,000.00 depending upon the size of the smallest feature in the pattern to be formed in the photo-resist layer on the wafer's surface. Consequently, a complete set of reticles needed for fabricating a single type of IC may cost several hundred thousand dollars. Correspondingly, the photolithography equipment which receives both the reticle and the wafer for exposing the photo-resist layer to short wavelength light is also comparatively expensive costing several million dollars.
A typical IC factory, commonly referred to as a “fab,” may include several different models of photolithography equipment from different manufacturers, or different models of photolithography equipment from the same manufacturer. While these differing models of photolithography equipment will all accept the same reticles used in manufacturing a single type of IC, previously there existed no standard holder for a single reticle or cassette for holding a set of reticles while reticles are automatically loaded into and removed from the photolithography equipment. That is, individual photolithography equipment manufacturers have arbitrarily selected unique configurations for holders and cassettes which carry reticles while reticles are loaded into and removed from photolithography equipment. Thus, worldwide presently there are in daily use in IC fabs reticle holders and cassettes having dozens of different, incompatible configurations.
In an effort to standardize reticle cassettes among the products of various photolithography equipment manufacturers, the Semiconductor Equipment and Materials International (“SEMI”) has adopted a standard, i.e. SEMI E100-0302, entitled “Specification for a Reticle SMIF Pod (RSP) Used to Transport and Store 6 Inch or 230 mm Reticles.” The SEMI E100-0302 standard is hereby incorporated by reference. As implied by the name of the SEMI standard, the configuration of RSP is an adaptation of a previously existing Standard Mechanical InterFace (“SMIF”) pod which is widely used in IC fabs for carrying 8-inch semiconductor wafers during wafer processing. While it appears likely that sometime in the future all photolithography equipment will accept the RSP for holding reticles while individual reticles are automatically loaded into and removed from the photolithography equipment, due to the presently existing large installed base of photolithography equipment such a situation is unlikely to occur in the immediately foreseeable future. Thus, for the foreseeable future a need will continue to exist for automatically moving reticles into and out of uniquely configured photolithography equipment.
Some photolithography tools used in patterning blank reticles accept and deliver cassettes which carry a number of reticles. Some IC fabrication photolithography tools, as contrasted with photolithography tools used in patterning blank reticles, provide storage within the tool for a relatively small inventory of reticles. Other IC fabrication photolithography tools are unable to store any reticles. However, whether an IC fabrication photolithography tool provides or lacks storage for an inventory of reticles, such tools are, in general, configured for receiving or delivering single reticles one at a time.
Because IC fabs store reticles in various differing ways when they are not being used for producing patterned photo-resist material on a wafer's surface, presently reticles are manually delivered to and received from IC fabrication photolithography tools. Manually exchanging reticles with IC fabrication photolithography tools insures that, regardless of how reticles are stored when they are not in use, they will be oriented properly in the tool for patterning photo-resist material on a wafer's surface.
As is well known to those skilled in the art of IC fabrication, contamination must be reduced as much as practicable, or even eliminated if possible, within an IC fab. For example, to reduce reticle contamination they may be stored with their patterned surface facing downward. However, proper focus within a photolithography tool may require reorienting the reticle's patterned surface. Properly designed equipment for automatically transferring reticles into and out of photolithography equipment which is capable of reorienting them can reduce or eliminate a possible source of contamination due to manual handling.
Devices used for exchanging with an IC processing tool cassettes which carry a number of semiconductor wafers have been used routinely for a number of years in IC fabs. U.S. Pat. Nos. 5,984,610 and 6,086,323 (“the '610 and '323 patents”), that are incorporated herein by reference, describe such equipment which is frequently identified by the name “pod opener.” Pod openers such as those described in these patents exchange with an IC processing tool a cassette carrying a number of semiconductor wafers that are initially enclosed within a SMIF pod. In general, a pod opener accepts a sealed SMIF pod and then opens the pod to expose any wafer cassette carried therein while constantly maintaining an ultra-clean environment around the cassette. After the pod opener exposes the wafer cassette, a robotic arm having an end effector adapted for gripping the cassette exchanges the cassette with an IC processing tool. In general, the semiconductor wafers are oriented horizontally when the cassette is enclosed within the SMIF pod. For that reason, whenever possible IC processing tools are designed to exchange horizontally oriented, individual semiconductor wafers with the wafer cassette. However, depending upon requirements of particular IC processing tools, as described in U.S. Pat. No. 5,885,045 the robotic arm and end effector included in a pod opener may be adapted for reorienting the cassette so the semiconductor wafers are oriented vertically. However, conventional pod openers are not usually equipped for handling and reorienting individual silicon wafers.